HIV/AIDS is regarded as one of the most globalized epidemics in human history. Yet its transmission patterns and the social responses to it have been diverse among various localities and peoples. By examining the masculine identity and life transitions among male youths, this article seeks to answer why the Nuosu people in high mountains of Sichuan Province in Southwest China has a disproportionately large number of heroin users and HIV positive individuals relative to other peoples since the 1990s.Based on over twenty months of ethnographic research in a rural community, this research posits these problems as an unfortunate consequence of a highly complex social process in which drastic political economic changes, cultural and ethnic conflicts, as well as rising individuality among the young are intertwined. This convoluted social process illustrates a distinct case of the so-call AIDS pandemic in a multicultural and multiethnic society.